Optical suppression of hyperfine-changing collisions in a sample of ultracold sodium atoms

We report the observation of hyperfine-changing collisions and the suppression of their rates in a sample of ultracold sodium atoms. We show that the suppression scheme used in the study of photoassociative ionization of Na [Phys. Rev. Lett. 76, 2033 (1996)] is also able to suppress the hyperfine-changing collisions, producing suppression up to 80%. The experimental results are compared with predictions of a simple Landau-Zener model and of a three-dimensional, multichannel close-coupling calculation. Both theories agree at low intensity, but at high intensity the experiment shows a saturation behavior that is qualitatively described only by the three-dimensional multichannel theory.